Vitamin A inhibits the growth and development of S-91 melanoma in BALB/c mice. The required dose for this effect is 3500 u/day for 5 days. Recently we observed that the Vitamin A treatment can be given after tumor inoculation. This is the first demonstration of the effectiveness of this compound in preventing growth of transplanted tumor cells. Congeners of Vitamin A are also effective, and possibly have an improved toxic/therapeutic ratio.